Automatic gain control



' INVENTOR WOLFGANG KAUTTER BY #QZ ATTORNEY Oct. 24-, 1933. w. KAUTTER AUTOMATIC GAIN CONTROL Filed llay 1'7, 1932 Patented Oct. 24, 1933 PATENT easier AUTOMATIC GAIN CONTROL Wolfgang Kautter, Berlin-Charlottenburg, Germany, assignor to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application May 17, 1932, Serial No. 611,871, and inGermany May 2, 1931 5 Claims. (01. 250-20) It is known in the prior art that in radio intelligence transmission and communication work, reception is considerably affected by fading phenomena whenever relatively short waves are 5 used. Attempts have been made in various quarters to compensate or equalize such variations in intensity caused as a consequence in the incoming field intensity by the intermediary of automatically acting regulators combined with the receiver apparatus. What has been used for this purpose are motor regulator means or auxiliary amplifier arrangements which are operating in dependence upon a quantity which is proportional to the average field intensity level at the receiving point. As suited for this purpose may be generally regarded the carrier Wave. Indeed,

.the regulator means have in the past been made preferably dependent upon the carrier wave. They react upon the gain of the receiver ar- '20 rangement in that the grid potential in the radio frequency stages, especially that of the input stage, is influenced thereby. Motor regulation means requirea considerable amount of circuit and circuit-changing means, and as a result they exhibit a great tendency toward disturbances in their function. Arrangements predicated for their operation upon supplementary amplifier means transfer or impress the variations in the carrier wave, which are represented most simply by the variations of the rectified current, by way of direct coupling upon the grid circuit of one or several radio frequency stages and influence the grid D. C. potential thereof. Owing to the galvanic coupling between the plate circuit of the regulating stage and the grid circuit of the radio frequency stage a special counter potential must be provided and applied to the grid circuit in order to compensate the plate potential in the former, and this results in difficulties in anlattempt at preserving a definite grid poten- Now, the said difficulties are obviated by the present invention. The field of application thereof is not limited by any means, in fact, it may include the entire wave-range nowadays used in communication work. But in practice it is primarily the broadcast wave-range that comes into consideration. Since in this case a practically constant modulation of the carrier wave is dealt with, this consequently involves a mean value. The special characteristic feature of the invention consists in that recourse is had to an auxiliary wave which is subject to the influence of the mean carrier-wave amplitude level and Which after being fed to a recti- T fier circuit is employed to regulate the grid D. C. potential of the radio frequency stages, particularly that of the input stage, in the desired sense. Inasmuch as the transfer is effected inductively the grid circuit is locked against the regulator device itself. Another feature of the invention may be regarded to reside in that a principle known from power engineering, 1. e., used for instance, in the Tirrill regulator, is here employed in order that the said auxiliary Wave Q5 may'becontrolled in dependence upon theaverage variations of the detector (D. C.) current. In this Way a particularly simple construction of the regulation device of the invention is insured. In addition, it is possible to combine the details of the embodiment hereinafter to be described by way of example in a distinct comparatively small accessory unit and to thus connect the latter with existing receiver apparatus. H15

In the drawing the part of the broadcast receiver set which is essential in connection with the basic idea of the invention is illustrated.

The broadcastreceiver scheme is supposed to contain a radio frequency amplifier stage com-mp prising tube 1 and constituting the input stage and in conjunction therewith a detector stage with tube 2. The tunable input circuit comprising the aerial conductor and the audio frequency amplifier have been omitted in the draw- '85 ing. 3 designates the oscillation tube of a thermionic generator designed to produce the auxiliary oscillation above referred to. The circuit of said auxiliary oscillation consists of the capacity 4' and the inductance of the transformer 7 reduced to the primary 5.

The insuing frequency of the oscillation lies above the audibility range. Energy is furnished from the plate potential source 8. Regeneration is insured by the capacity 9 and by virtue of the internal coupling inside the tube 3. The oscillationsare transferred to a secondary circuit which con- -sists of the Winding 6 of the transformer 7, the

rectifier l5 and the capacities and resistances 16, 17, 18 and 19. By way of the rectifier 15 theil capacities 16, 17 are charged, and by the latter and the resistances 18, 19 the size of the negative D. C. potential of tube 1 is governed. 'With increasing amplitude of oscillation of the generator valve the grid D. C. potential will grow whereby a decrease in the radio frequency amplification (gain) is effected. Now, it would be possible that the thermionic generator in dependence upon the average carrier-wave amplitude, especially uponthe drop .of current 00-1 oscillator 3.

casioned thereby in the plate circuit of the audion tube 2 is controlled in such a fashion that a change in amplitude will cause a proportional change, or a change occurring in accordance with a certain law, in the amplitude of the waves of the thermionic generator. This case would correspond to a modulation scheme of the thermionic generator. However, it would be feasible also to choose another procedure which would flow from the Tirrill principle hereinbefore mentioned and which offers essential advantages, with a view to insure a simple form of construction of the circuit arrangements. In this connection the following consideration must be made: The oscillation scheme of the thermionic generator should be so chosen and dimensioned that, in the presence of a definite value of the grid D. 0. potential, initiation of the oscillations and at a value only a little below the former discontinuance of the oscillations will be caused; and this interval in the potential should correspond to the admissible variation of the mean carrier-wave amplitude.

In the embodiment exemplified by the drawing the grid D. C. potential of the thermionic generator and its dependence upon the carrier wave is conditioned by the plate circuit of the detector tube 2. The plate of the same is united by way of the'primary winding of transformer 10 and the resistance 11 with the plate potential source 12 which is grounded at the other end. Re-

sistance 11 and capacity 14 represent filter means known in the art and they do not rep resent any additional elements forming part of this invention. In parallel relation to the source of potential 12 is a voltage divider 13 whose top is united with the filament of the The grid of this tube is connected by way of the leak resistance 20 with the connection between resistance 11 and the primary winding of 10. The tap of the voltage divider shall be supposed to be so set that in the presence of the normal carrier-wave and of a corresponding plate current of the audion tube 2, initiation of the oscillations in tube 3 will just be caused. As a result the condenser 17 will be charged up and there is brought about a reduction in the gain which in turn entails a growth in the plate current of the detector tube. As a result the grid D. C. potential of the oscillator tube 3 drops again to the value where the oscillations stop. Thereafter discharge of capacity 17 occurs by way of the resistance 19, amplification of the radio frequency stage being then raised again with a consequent renewed decrease in the plate current of the detector tube until the auxiliary oscillation is caused to be initiated again. The period of this phenomenon is governed by the dimensions of the capacities and resistances 16, 17, 18, 19. The

. same is kept low compared with the variations The regulikely to be occasioned by fadings. lative variations are so low that they will not disturb reception in any way whatever. If, then, the mean carrier-wave amplitude level as a result of fading decreases abruptly so that the plate D. C. of the detector tube will rise the D. 0. potential of the oscillator becomes more negative. As a result the capacity 17 will be dis charged by way of resistance 19 or the grid D. C. potential at the tube will be raised in a positive sense and the gain raised, until the tube 3 resumes oscillating. The regulatory cycle is thereupon continued in a way as hereinbefore described, but now about a mean value of the grid D. C. potential of tube 1 which is slightly higher. The inverse action will be produced whenever the field intensity at the receiving point rises suddenly and when the detector current decreases suddenly to a marked degree. In that case the oscillations of tube 3 set in sooner and react upon the input circuit until, as a consequence of an increased counter potential at the grid, the desired average amplitude of the a consequence of the rather high fall of potential across the resistance 11 and the fixed counter potential of the voltage divider 13, the

control of the oscillation circuit is made rather sensitive. When the tap on the voltage divider 13 is shifted more towards the positive pole of the source of potential 12, the oscillator is not able at all to oscillate any more, and this a circumstance that is very essential in the adjustment of transmitters. If a given fading action falls below the value corresponding to the range of the regulator arrangement, especially to the proportional shift of the grid circuit in 1 5 the input stage, then the difference will manifest itself as a real fading action or decrease in volume.

The shape of the curve of the wave furnished from the oscillator 3 is immaterial. 15 for the sake of simplicity may be of the dry (electrolytic) type. The network (filter) constituted by the capacities 16, 17 and resistance 13, 19, which governs the period of the regulator action, at the same time fulfills a further pour- 15 pose; for according to another object of the invention the same is so dimensioned that smoothing of the pulsating D. C. fedtherein is effected. As a result the potential acting across the capacity 17 is perfectly continuous, 'disre-* 20 garding such alterations as are inherent in the regulator action.

' For the production of the auxiliary wave recourse may be had also to other regenerative circuit schemes,'in fact, all such means may be employed therefor as are feasible generally in wave generationby means of tubes. It is furthermore feasible to use electromagnetic machines, electro-mechanical oscillation systems and the like. All of these arrangements must' have one common feature, namely, that they generate oscillations and that they are influenceable in a way as hereinbefore described. For the control of all of the said auxiliary-wave systems any D/C. circuit is suited which is capable of being influenced by the carrier wave.

I claim: 1. In an automatic volume control system for a radio receiver of the type which includes a radio frequency amplifier and a detector, an oscillator circuit including a tube having its input electrodes connected to the anode circuit of the detector, a rectifier, means for impressing the oscillator tube output upon said rectifier, and

additional means for impressing the rectifier direct current output upon the input electrodes of said emplifier to control the gain of the latter.

2. In an automatic volume control system for a radio receiver of the type which includes a radio frequency amplifier and a detector, an 15a to an arrangement of 90 The rectifier 110 oscillator circuit including a tube having itsvinput electrodes connected to the anode circuit of the detector, a rectifier, inductive means for impressing the oscillator tube output upon said rectifier, and additional means for impressing the rectifier direct current output upon the input electrodes of said amplifier to control the gain of the latter.

3. In an automatic volume control system for a radio receiver of the type which includes a radio frequency amplifier and a detector, an oscillator circuit including a tube having its input electrodes connected to the anode circuit of the detector, a rectifier, means for impressing the oscillator tube output upon said rectifier, and additional means, including a filter network, for impressing the rectifier direct current output upon the input electrodes of said amplifier to control the gain of the latter.

4. In an automatic volume control system for a radio receiver of the type which includes a radio frequency amplifier and a leaky grid condenser detector, an oscillator circuit including a tube having its input electrodes connected to the anode circuit of the detector, a rectifier, means for impressing the oscillator tube output upon said rectifier, and additional means for impressing the rectifier direct current output upon the input electrodes of said amplifier to control the gain of the latter.

5. In an automatic volume control system for a radio receiver of the type which includes a radio frequency amplifier and a detector, an oscillator circuit including a tube having its input electrodes connected to the anode circuit of the detector, means, common to the detector anode circuit and the oscillator tube input electrodes, for adjusting the initial potential difference between the oscillator tube input electrodes, a rectifier, means for impressing the oscillator tube output upon said rectifier, and additional means for impressing the rectifier direct current output upon the input electrodes of said amplifier to control the gain of the latter.

WOLFGANG KAUTTER. 

